1. Field of the Invention
This invention relates to a developing assembly, a process cartridge and an image-forming method which are usable in recording processes utilizing electrophotography or electrostatic recording.
2. Related Background Art
Electrophotographic processes are disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 (U.S. Pat. No. 3,666,363), Japanese Patent Publication No. 43-24748 (U.S. Pat. No. 4,071,361) and so forth. In general, copies or prints are obtained by forming an electrostatic latent image on an electrostatic latent image bearing member (photosensitive member) by various means utilizing a photoconductive material, subsequently developing the electrostatic latent image by the use of a developer (hereinafter often referred to as simply “toner”) to form a toner image, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by the action of heat, pressure, solvent vapor, or heat-and-pressure.
In recent years, in addition to conventional copying machines, equipments making use of electrophotography have become various, as exemplified by printers and facsimile machines. Developing systems are roughly grouped into a two-component developing system making use of carrier particles and a one-component developing system making use of no carrier particles. The one-component developing system is one in which a toner is triboelectrically charged chiefly by the friction of the toner with a triboelectric charging member, and is roughly grouped into a one-component magnetic developing system in which magnetic particles are incorporated in toner particles and a developer is carried and transported by the action of magnetic force and a one-component non-magnetic developing system in which, without use of any magnetic particles, a developer is carried on a developer-carrying member by the action of triboelectric charges of the developer. In the one-component magnetic developing system, without use of any colorant such as carbon black, magnetic particles may be made to serve also as the colorant.
The two-component developing system requires a device with which the concentration of toner is detected to supply the toner in a necessary quantity, because carrier particles such as glass beads or iron powder are necessary in order to impart electric charges to the toner by their friction with the toner or because the concentration of toner in the developer must be kept constant. Accordingly, its developing assembly is large and heavy and also has complicate construction. The two-component developing system also tends to cause adhesion of toner components to the carrier (i.e., toner-spent), and hence the carrier must frequently be replaced. In this regard, the one-component developing system does not require any carrier and such complicate construction, and can make the developing assembly itself compact, small-size and light-weight. In addition, it does not require any replacement of carriers, and hence makes maintenance service unnecessary over a long period of times. On the other hand, the one-component magnetic developing system is difficult to employ in color development because pitch-black magnetic particles are used in toners, whereas the two-component developing system enables control of delicate development condition by means of the concentration detection device and hence is preferably used in color development.
Without regard to the difference between the one-component type and the two-component type, methods are also proposed in which an inorganic fine powder is added to toner particles as an agent externally added (an external additive), and are put into wide use.
For example, Japanese Patent Applications Laid-Open No. 5-66608, No. 4-9860 and so forth disclose a method in which an inorganic fine powder having been subjected to hydrophobic treatment or an inorganic fine powder having been subjected to hydrophobic treatment and thereafter further treatment with a silicone oil is added; and Japanese Patent Applications Laid-Open No. 61-249059, No. 4-264453 and No. 5-346682, a method in which a hydrophobic-treated inorganic fine powder and a silicone-oil-treated inorganic fine powder are used in combination and added.
Methods in which conductive fine particles are externally added to developers as the external additive are also proposed in a large number. For example, carbon black as conductive fine particles is widely known to be used as an external additive for adhering or sticking to toner particle surfaces, for the purpose of providing conductivity to toners or controlling any excess charging of toners to make their triboelectric distribution uniform. Also, Japanese Patent Applications Laid-Open No. 57-151952, No. 59-168458 and No. 60-69660 disclose external addition of conductive fine particles such as tin oxide, zinc oxide and titanium oxide, respectively, to high-resistance magnetic toner. In Japanese Patent Application Laid-Open No. 56-142540, also proposed is a developer in which conductive magnetic particles such as iron oxide, iron powder or ferrite are added to a high-resistance magnetic toner to make the conductive magnetic particles accelerate the induction of electric charges to the magnetic toner so that both developing performance and transfer performance can be achieved. Further, Japanese Patent Applications Laid-Open No. 61-275864, No. 62-258742, No. 61-141452 and No. 2-120865 disclose addition of graphite, magnetite, polypyrrole conductive particles or polyaniline conductive particles to toners.
Various methods are also known in respect of methods of forming electrostatic latent images on latent-image-bearing members such as electrophotographic photosensitive members and electrostatic recording dielectrics. For example, in electrophotography, a method is common in which as a latent-image-bearing member a photosensitive member utilizing a photoconductive material is uniformly charged to the necessary polarity and potential and thereafter the surface of this photosensitive member is subjected to image pattern exposure to form an electrical latent image.
Corona charging assemblies (corona dischargers) have widely been used as charging assemblies for uniformly charging (also inclusive of charge eliminating) latent-image-bearing members to the necessary polarity and potential.
The corona charging assembly is a non-contact type charging assembly. It has a discharge electrode such as a wire electrode and has a shield electrode which surrounds the discharge electrode. Its discharge opening is provided opposingly to and in non-contact with a charging object member latent-image-bearing member, where a high voltage is applied across the discharge electrode and the shield electrode to cause discharge electric current (corona shower) to take place, to which the surface of the latent-image-bearing member is exposed to charge the latent-image-bearing member surface to the intended polarity and potential.
In recent years, contact charging assemblies are proposed in a large number as charging assemblies for charging object members such as latent-image-bearing members because of their advantages of lower ozone generation and lower power consumption than the corona charging assemblies, and have been put into practical use.
The contact charging assembly is an assembly in which a conductive charging member of a roller type (charging roller), a fur brush type, a magnetic-brush type or a blade type is brought into contact with a charging object member such as an image-bearing member and a stated charging bias is applied to this contact charging member or contact charging assembly to charge the surface of the charging object member to the stated polarity and potential.
The charging mechanism (charging principle) of contact charging mixedly involves two types of charging mechanisms, which are (1) discharge charging mechanism and (2) direct-injection charging mechanism. Their characteristics are brought out depending on which mechanism governs the other.
(1) Discharge Charging Mechanism of Contact Charging
This is the mechanism in which the charging object member surface becomes charged by the phenomenon of discharge caused at any microscopic gap(s) to be formed between the contact charging member and the charging object member. The discharge charging has a certain discharge threshold value between the contact charging member and charging object member, and hence a voltage greater than the charge potential must be applied to the contact charging member. Though generated in a remarkably smaller quantity than that in corona charging assemblies, a discharge product is inevitably generated in principle, and hence difficulties due to active ions such as ozone are unavoidable.
(2) Direct-Injection Charging Mechanism of Contact Charging
This is a system in which electric charges are directly injected from the contact charging member into the charging object member to charge the charging object member surface electrostatically. This is also called direct charging, or injection charging, or electric-charge injection charging. Stated more specifically, this is a method in which a medium-resistance contact charging member is kept in contact with the charging object member surface to inject electric charges directly to the surface of the charging object member not through any discharge phenomenon, in short, without using any discharge mechanism basically. Hence, even if the voltage applied to the charging object member is not higher than the discharge threshold value, the charging object member can be charged to the potential corresponding to the applied voltage. This charging system is not accompanied with the generation of active ions such as ozone, and hence any difficulties that may be caused by discharge products does not occur. However, because of direct injection charging, the contact performance of the contact charging member on the charging object member has a great influence on the charging performance. Accordingly, in order to afford construction in which the contact charging member comes into contact with the charging object member more highly frequently, the contact charging member is required to have the construction such that it has closer contact points and has much difference in speed from the charging object member.
In the contact charging assembly, a roller charging system making use of a conductive roller (charging roller) is preferable in view of the stability of charging, and is put into wide use.
The charging mechanism in conventional roller charging is predominantly governed by the above (1) discharge charging mechanism. The charging roller is formed using a conductive or medium-resistance rubber material or foam. In some roller, such a rubber material or foam is provided in layers to attain the desired characteristics.
The charging roller is provided with an elasticity in order to ensure the state of a uniform contact between it and the charging object member. For this reason, it has a great frictional resistance, and in many cases it is driven in follow-up with, or at some difference in speed from, the rotation of the charging object member. Hence, any attempt of direct-injection charging may inevitably cause a decrease in absolute chargeability, a contact unevenness due to shortage in contact performance and roller shape and a charging unevenness due to any deposits on the charging object member.
FIG. 1 is a graph showing examples of charging efficiency of contact charging in electrophotography. The bias voltage applied to the contact charging member is plotted as abscissa, and the charge potential of the charging object (hereinafter “photosensitive member”), obtained there, is plotted as ordinate.
Charge characteristics in the case of roller charging are represented by A. That is, the surface potential of the photosensitive member begins to rise after the applied voltage exceeds a threshold value of about −500 V, and, at voltages higher than such threshold value, the photosensitive member surface potential increases linearly at a slope of 1 with respect to the applied voltage. This threshold value voltage is defined as charging start voltage Vth. Accordingly, when the photosensitive member is charged to −500 V, it is common to employ a method in which a DC voltage of −1,000 V is applied, or, in addition to the charging voltage of −500 V, an AC voltage of, e.g., a peak-to-peak voltage of 1,200 V is applied so as to provide a potential difference larger than the discharge threshold value, to converge the photosensitive member potential to the charge potential.
In order to obtain a photosensitive member surface potential Vd that is required in electrophotography, a DC voltage of “Vd+Vth”, what is higher than is necessary, must be applied to the charging roller. The charging performed by applying only a DC voltage to the contact charging member in this way is called “DC charging”.
In the DC charging, however, it has been difficult to control the potential of the photosensitive member at the desired value because the resistance value of the contact charging member varies depending on environmental variations and also because the Vth varies with changes in layer thickness caused by the abrasion of the photosensitive member.
Accordingly, in order to achieve more uniform charging, as disclosed in Japanese Patent Application Laid-Open No. 63-149669, “AC charging system” may be used which is a method of applying to the contact charging member a voltage formed by superimposing an AC component having a peak-to-peak voltage of 2×Vth or above, on a DC voltage corresponding to the desired Vd. This method aims at a potential-leveling effect which is attributable to AC, where the potential of the charging object member converges on Vd, the middle of a peak of AC potential, and is by no means affected by external disturbance such as environmental variations.
However, even in such contact charging assemblies, its fundamental charging mechanism employs the phenomenon of discharge from the contact charging member to the photosensitive member. Hence, as stated previously, the voltage applied to the contact charging member is required to be the value higher than the desired surface potential of the photosensitive member, and the ozone may come therefrom at least are a very small level. Also, when the AC charging is performed in order to achieve uniform charging, the ozone may more be generated, the electric field of AC voltage may cause a vibrating noise (AC charging sound) between the contact charging member and the photosensitive member, and any discharging may remarkably cause deterioration or the like of the surface of the photosensitive member. These have come into additional question.
The fur brush charging is one in which, using as a contact charging member a member having a conductive-fiber brush portion (a fur brush charging assembly), the conductive-fiber brush portion is brought into contact with a photosensitive member as the charging object, and a stated charging bias is applied to the conductive-fiber brush portion to charge the surface of the photosensitive member electrostatically to the stated polarity and potential. In this fur brush charging, too, its charging mechanism may predominantly be governed by the above (1) discharge charging mechanism.
For the fur brush charging assembly, a fixed type and a roll type have been put into practical use. One in which medium-resistance fibers formed in a folded pile on a base cloth have been bonded to an electrode is the fixed type. The roll type is formed by winding pile around a mandrel. Those having a fiber density of about 100 fibers/mm2 are obtained relatively with ease, but are still insufficient for contact performance in order to perform well uniform charging by direct-injection charging. In order to perform well uniform charging by direct-injection charging, the fur brush charging assembly must be made to have a velocity differential from that of the photosensitive member; the difference being so large as to make machine construction difficult. This is not realistic.
Charge characteristics of this fur brush charging at the time of application of DC voltage are as shown by B in FIG. 1. Hence, in the case of fur brush charging, too, in both the fixed type and the roll type, the charging is performed under application of a high charging bias voltage in many cases to utilize a phenomenon of discharging.
In contrast to these, the magnetic-brush charging is one in which, using as a contact charging member a member having a magnetic-brush portion (a magnetic-brush charging assembly) formed by confining conductive magnetic particles magnetically by means of a magnet roll, the magnetic-brush portion is brought into contact with a photosensitive member as the charging object, and a stated charging bias is applied to charge the surface of the photosensitive member electrostatically to the stated polarity and potential. In the case of this magnetic-brush charging, its charging mechanism is predominantly governed by the above (2) direct-injection charging mechanism.
As the conductive magnetic particles of which the magnetic-brush portion is constituted, those having particle diameter of from 5 μm to 50 μm may be used, and a sufficient velocity differential from that of the photosensitive member may be provided, whereby almost uniform direct-injection charging can be performed.
Charge characteristics of the magnetic-brush charging at the time of application of DC voltage are shown by C in FIG. 1. As shown in FIG. 1, it is possible to attain a charge potential substantially proportional to the applied bias voltage.
The magnetic-brush charging, however, may also cause a difficulty that the conductive magnetic particles constituting the magnetic-brush portion come off to adhere to the photosensitive member. Thus, it is sought to provide an assembly for simple, stable and uniform charging, which can be operated by the direct-injection charging mechanism causing substantially no discharge products such as ozone and achievable of uniform charging at a low applied voltage.
Especially in recent years, from the viewpoint of resource saving and waste reduction and in the sense of effective utilization of toners (developers), an image-forming method which does not bring any transfer residual toner, i.e., waste toner is desired. In the past, in general, after a latent image has been developed with a toner into a visible image (toner image) and the toner image has been transferred to a recording medium such as paper, the toner having remained on the latent-image-bearing member without being transferred to the recording medium is removed by a cleaning means (cleaner), and is transported and put away as waste toner into a waste toner container. Through such a cleaning step, the step of forming images is repeated. Such an image-forming apparatus has been in side used.
In this cleaning step, blade cleaning, fur brush cleaning, roller cleaning and so forth have conventionally been used. Any of these methods are those in which the transfer residual toner is mechanically scraped off or is dammed up and then transported to the waste toner container. Accordingly, with a growing tendency toward resource saving and environmental conservation, it is being demanded to establish the system of reusing or disposing of the waste toner after the waste toner stored in the waste toner container has been collected. Meanwhile what is called the toner reuse, in which the toner collected at the cleaning step is circulated into the developing assembly and reused, has been put into practical use, in which, after a latent image on a latent-image-bearing member is developed with a toner to form a toner image as a visible image and the toner image is transferred to a recording medium such as paper, any toner having remained on the latent-image-bearing member without being transferred to the recording medium is removed by cleaning by various methods, and this toner is circulated into a developing assembly and reused. There, however, has been a problem that pressing a cleaning member against the latent-image-bearing member surface causes the latent-image-bearing member to wear to make the latent-image-bearing member have a short lifetime. Also, when viewed from the standpoint of apparatus, the image-forming apparatus must be made larger in size in order to provide such a toner reuse assembly and a cleaning assembly. This has been a bottleneck in attempts to make apparatus compact.
As a countermeasure therefor, as a system which does not bring any waste toner, also proposed is a technique called a cleaning-at-development or cleanerless system. Conventional techniques concerning the cleaning-at-development or cleanerless system are, as disclosed in Japanese Patent Application Laid-Open No. 5-2287, focused on positive memory or negative memory appearing on images because of an influence of the transfer residual toner on images. However, in these days where electrophotography is utilized on and on, it has become necessary to transfer toner images to various recording mediums. In this sense, such techniques have not been satisfactory for various recording mediums.
The related art having disclosed techniques concerning the cleanerless system is seen in Japanese Patent Applications Laid-Open No. 59-133573, No. 62-203182, 63-133179, No. 64-20587, No. 2-302772, No. 5-2289, No. 5-53482 and No. 5-61383. These, however, neither mention any desirable image-forming methods nor refer to how the toner be constituted.
As developing systems in which the cleaning-at-development or cleanerless system is preferably applied, having basically no cleaning assembly, it has ever been considered essential for the system to be so made up that the latent-image-bearing member surface is rubbed with the toner and toner-carrying member. Accordingly, studies have largely been made on contact developing systems in which the toner or developer comes into contact with an latent-image-bearing member. This is because, in order to collect the transfer residual toner in a developing means, it is considered advantageous for the system to be so made up that the toner or developer comes into contact with and rub the latent-image-bearing member. However, in the cleaning-at-development or cleanerless process making use of a contact development system, its long-term service tends to cause deterioration of toner, deterioration of toner-carrying member surface and deterioration or wear of latent-image-bearing member surface, but any satisfactory solution has not been made for running performance. Accordingly, it has been sought to provide a cleaning-at-development system according to a non-contact developing system.
Here, think about an instance in which the contact charging method is applied in the cleaning-at-development method or cleanerless image-forming method. In the cleaning-at-development method or cleanerless image-forming method, any cleaning member is used, and hence the transfer residual toner left remaining on the latent-image-bearing member comes into contact with the contact charging member as it is, and adhere to or migrate into this contact charging member. Also, in the case of the charging method predominantly governed by the discharge charging mechanism, the transfer residual toner may come to greatly adhere to the contact charging member because of any toner deterioration due to discharge energy. Where any insulating toner commonly used has adhered to or migrated into the contact charging member, a lowering of charging performance may occur.
In the case of the charging system predominantly governed by the discharge charging mechanism, this lowering of charging performance may occur abruptly around the time when a toner layer having adhered to the contact charging member surface comes to have a resistance which may obstruct the discharge voltage. On the other hand, in the case of the charging system predominantly governed by the direct-injection charging mechanism, the uniform charging performance on the charging object member may lower where the transfer residual toner having adhered to or migrated into the contact charging member has lowered the probability of contact between the contact charging member surface and the charging object member.
This lowering of uniform charging performance on the charging object member may lower the contrast and uniformity of electrostatic latent images after imagewise exposure to cause difficulties such as a decrease in image density and an increase in fog. occur seriously.
In this cleaning-at-development system or cleanerless image-forming method, the point is that the charge polarity and charge quantity of the transfer residual toner on the photosensitive member is controlled so that the transfer residual toner can stably be collected in the step of development and the collected toner may not make the developing performance poor. Accordingly, the charge polarity and charge quantity of the transfer residual toner on the photosensitive member is controlled by means of the charging member. This will be described specifically taking the case of a commonly available laser beam printer.
In the case of reverse development making use of a charging member for applying a voltage with negative polarity, a negatively chargeable photosensitive member and a negatively chargeable toner, in the transfer step thereof the image rendered visible is transferred to the recording medium by means of a transfer member to which a voltage with positive polarity is applied. The charge polarity of the transfer residual toner varies from positive to negative depending on, for example, the relation between kinds of recording mediums (differences in thickness, resistance, dielectric constant and so forth) and the areas of images. However, when the photosensitive member is charged with the charging member having a negative polarity, the charge polarity of the transfer residual toner can uniformly be adjusted to the negative side together with the photosensitive member surface even if the polarity of the transfer residual toner has been shifted to the positive side in the transfer step. Hence, when the reversal development is employed as the developing system, the transfer residual toner, which stands negatively charged, remains at light-area potential areas to be developed by toner. On the other hand, the toner present at dark-area potential areas not to be developed by toner is attracted toward the toner carrying member in relation to the development electric field and is collected without remaining on the photosensitive member having a dark-area potential. That is, the cleaning-at-development or cleanerless image-forming method can be established by controlling the charge polarity of transfer residual toner simultaneously with the charging of the photosensitive member by means of the charging member.
However, where the transfer residual toner has adhered to or migrated into the contact charging member beyond the contact charging member's capacity to control toner's charge polarity, it becomes impossible to uniformly adjust the charge polarity of the transfer residual toner, making it difficult to collect the toner in the step of development. Also, even where the transfer residual toner has been collected on the toner-carrying member by mechanical force such as rubbing, the transfer residual toner may adversely affect the triboelectric chargeability of toner on the toner-carrying member, resulting in a lowering of developing performance, unless the charge of the transfer residual toner has not uniformly been adjusted. More specifically, in the cleaning-at-development or cleanerless image-forming method, the charge control performance at the time the transfer residual toner passes the charging member and the manner in which the transfer residual toner adheres to or migrates into the charging member are closely concerned with the running performance and image quality characteristics.
In the cleaning-at-development image-forming method, cleaning-at-development performance can be improved by improving charge control performance required when the transfer residual toner passes the charging member. As a proposal therefor, Japanese Patent Application Laid-Open No. 11-15206 discloses an image-forming method making use of a toner having toner particles containing specific carbon black and a specific azo type iron compound and having inorganic fine powder. It is also proposed, in the cleaning-at-development image-forming method, to improve cleaning-at-development performance by reducing the quantity of transfer residual toner, using a toner having a superior transfer efficiency the shape factors of which have been specified. However, the contact charging used here also applies the discharge charging mechanism, which is not the direct injection charging mechanism, and has the above problem ascribable to the discharge charging. Moreover, these proposals may be effective for keeping the charging performance of the contact charging member from lowering because of the transfer residual toner, but can not be expected to be effective for actively improving the charging performance.
In addition, among commercially available electrophotographic printers, cleaning-at-development image-forming apparatus are also available in which a roller member coming into contact with the photosensitive member is provided between the transfer step and the charging step so that the performance of collecting the transfer residual toner at development can be assisted or controlled. Such image-forming apparatus have good cleaning-at-development performance and the waste toner can sharply be reduced, but involve a high cost and may damage the advantage inherent in the cleaning-at-development system also in view of compact construction.
In order to prevent uneven charging to effect stable and uniform charging, the contact charging member may be coated with a powder on its surface coming into contact with the surface of the member to be charged. Such constitution is disclosed in Japanese Patent Publication No. 7-99442. However, the contact charging member (charging roller) is so constructed as to be follow-up rotated as the charging object member (photosensitive member) is rotated (without no velocity differential drive), and hence may remarkably less cause ozone products compared with corona charging assemblies such as Scorotron. However, the principle of charging is still chiefly the discharge charging mechanism like the case of the roller charging described previously. In particular, a voltage formed by superimposing AC voltage on DC voltage is applied in order to attain more stable charging uniformity, and hence the ozone products caused by discharging may more greatly occur. Accordingly, when the apparatus is used over a long period of time, difficulties such as smeared images due to ozone products tend to come out. Moreover, when the above construction is applied in cleanerless image-forming apparatus, any inclusion of the transfer residual toner makes it difficult for the powder coated, to stand adhered uniformly to the charging member, so that the effect of carrying out uniform charging may lower.
Japanese Patent Application Laid-Open No. 5-150539 also discloses that, in an image-forming method making use of contact charging, at least image-developing particles and conductive fine particles having an average particle diameter smaller than that of the image-developing particles are contained in a toner in order to prevent any charging obstruction which may be caused when toner particles or silica particles having not completely be removed by blade cleaning come to adhere to and accumulate on the surface of the charging means during repetition of image formation for a long time. However, the contact charging used here, or proximity charging, applies the discharge charging mechanism, which is not the direct injection charging mechanism, and has the above problem ascribable to the discharge charging. Moreover, when this construction is applied in the cleanerless image-forming apparatus, nothing is taking into consideration about any of the influence on charging performance that is exercised when the conductive fine particles and transfer residual toner pass the charging step in a larger quantity than the apparatus having a cleaning mechanism, the influence on the collection of these large-quantity conductive fine particles and transfer residual toner in the developing step, and the influence on developer's developing performance that is exercised by the conductive fine particles and transfer residual toner thus collected. Furthermore, when the direct injection charging mechanism is applied in the contact charging, the conductive fine particles can not be fed to the contact charging member in necessary quantity to cause faulty charging due to the influence of the transfer residual toner.
In the proximity charging, it is also difficult to uniformly charge the photosensitive member because of the large-quantity conductive fine particles and transfer residual toner, and the effect of leveling patterns of the transfer residual toner can not be obtained, to cause pattern ghost because the transfer residual toner may shut out pattern-imagewise exposure light. In-machine contamination due to developer may further occur when a power source is instantaneously turned off or paper jam occurs during image formation.
As countermeasures for these, Japanese Patent Application Laid-Open No. 10-307456 discloses an image-forming apparatus in which a developer containing toner particles and conductive charge-accelerating particles having particle diameter which is ½ or less of the particle diameter of toner is applied in a cleaning-at-development image-forming method making use of the direct injection charging mechanism. According to this proposal, a cleaning-at-development image-forming apparatus can be obtained which can sharply reduce the quantity of waste toner and is advantageous for making the apparatus compact at a low cost, and good images are obtainable without causing any faulty charging and any shut-out or scattering of imagewise exposure light. It, however, is sought to make further improvement.
Japanese Patent Application Laid-Open No. 10-307421 also discloses an image-forming apparatus in which a developer containing conductive particles having particle diameter which is {fraction (1/50)} to ½ of the particle diameter of the toner is applied in a cleaning-at-development image-forming method making use of the direct injection charging mechanism and the conductive particles are made to have a transfer accelerating effect.
Japanese Patent Application Laid-Open No. 10-307455 still also discloses that, a conductive fine powder is controlled to have particle diameter not larger than the size of one pixel of constituent pixels, and the conductive fine powder is controlled to have particle diameter of from 10 nm to 50 μm in order to attain better charging uniformity.
Japanese Patent Application Laid-Open No. 10-307457 discloses that, taking account of the characteristics of human visual sensation, conductive fine particles are controlled to have particle diameter of about 5 μm or less, and preferably from 20 nm to 5 μm, in order to make any influence of faulty charging on images visually recognizable with difficulty.
Japanese Patent Application Laid-Open No. 10-307458 also discloses that a conductive fine powder is controlled to have particle diameter not larger than the particle diameter of a toner to thereby prevent the conductive fine powder from obstructing the development by the toner at the time of development or prevent development bias from leaking through the conductive fine powder. At the same time, it discloses a cleaning-at-development image-forming method which makes use of the direct injection charging mechanism and in which the conductive fine powder is controlled to have particle diameter larger than 0.1 μm to thereby eliminate a difficulty that the conductive fine powder may become buried in the image-bearing member to shut out imagewise exposure light, thus superior image recording can be materialized. It, however, is sought to make further improvement.
Japanese Patent Application Laid-Open No. 10-307456 discloses a cleaning-at-development image-forming apparatus in which a conductive fine powder is externally added to toner particles so that the conductive fine powder contained in the toner may adhere to an image-bearing member in the step of development, at least at a contact zone between a flexible contact charging member and the image-bearing member, and may remain and be carried on the image-bearing member also after the step of transfer so as to stand between them, to thereby obtain good images without causing neither faulty charging nor shut-off of imagewise exposure light. In this proposal, however, there is room for further improvement in stable performances required when the apparatus are repeatedly used over a long period of time and in performances required when toner particles having a small particle diameter are used in order to achieve a higher resolution.
External addition of conductive particles whose average particle diameter has been specified is also proposed. For example, in Japanese Patent Application Laid-Open No. 9-146293, a toner is proposed in which a fine powder A with an average particle diameter of from 5 nm to 50 nm and a fine powder B with an average particle diameter of from 0.1 μm to 3 μm are used as external additives, and have been made to adhere to toner base particles with particle diameters of from 4 μm to 12 μm, more strongly than a specified extent. This intends to make small the proportion of fine powder B standing liberated and those coming off the toner base particles. In Japanese Patent Application Laid-Open No. 11-95479, also proposed is a toner containing conductive silica particles whose particle diameter has been specified and an inorganic oxide having been made hydrophobic. This is nothing but what aims at the action attributable to the conductive silica particles by which action any electric charges accumulated in the toner in excess are leaked to the outside.
Many proposals are also made in which the particle size distribution and particle shape of toners have been specified. In recent years, as disclosed in Japanese Patent No. 2862827, there is a proposal in which particle size distribution and circularity measured with a flow type particle image analyzer have been specified. As proposals in which the particle size distribution and particle shape of toners have been specified taking account of any influence of external additives, for example, Japanese Patent Application Laid-Open No. 11-174731 discloses a toner having an inorganic fine powder A of 10 nm to 400 nm in average length the circularity of which has been specified and a non-spherical inorganic fine powder B. This proposal intends to keep the inorganic fine powder A from being buried in toner base particles in virtue of the spacer effect attributable to the non-spherical inorganic fine powder B. In Japanese Patent Application Laid-Open No. 11-202557, too, a proposal is made on specifying the particle size distribution and circularity of toners. This proposal is aimed at prevention of a trailing phenomenon by making the density high in respect of toner particles which have participated in development as a toner image, and at improvement in the storage stability of toners in an environment of high temperature and high humidity.
In Japanese Patent Application Laid-Open No. 11-194530, a toner is further proposed which has an external-additive fine powder A with particle diameter of from 0.6 μm to 4 μm and an inorganic fine powder B and whose particle size distribution has been specified. This intends to prevent the toner from deteriorating because of any inorganic fine powder B buried in toner base particles, in virtue of the presence of the external-additive fine powder A between them. Thus, nothing is taken into account in respect of any adhesion of the external-additive fine powder A to, or liberation from, the toner base particles. In Japanese Patent Application Laid-Open No. 10-83096, proposed is a toner comprising spherical resin particles in which a colorant has been enclosed and to the particle surfaces of which fine silica particles have been added. This intends to endow toner particle surfaces with conductivity to enable swift movement and exchange of electric charges across the toner particles and to improve the uniformity of triboelectric charging of the toner.
Meanwhile, approach has also been made from developers in order to establish the image-forming method having the step of injection charging, the cleaning-at-development image-forming method or the cleanerless image-forming method, i.e., in order to impart optimum electric charges to the developers (toners).
Conventionally, in image-forming apparatus of an electrophotographic system for example, an electrostatic latent image is formed on a latent-image-bearing member comprising an electrophotographic photosensitive member, and the latent image is developed by means of a developing assembly. The developing assembly has a developing sleeve serving as a developer-carrying member on which the developer is held and transported.
The surface of this developing sleeve is made to have a rough surface with unevenness (hills and dales) for the sake of its performance of transporting the developer (transport performance). Formerly, as disclosed in Japanese Patent Application Laid-Open No. 54-79043 for example, knurl grooves chiefly in respect of developing sleeves for two-component developers and, as disclosed in Japanese Patent Application Laid-Open No. 55-26526, blast treatment chiefly in respect of developing sleeves for one-component developers are known in the art.
In the case of blast-treated developing sleeves, the surface unevenness tends to become worn and lessen as a result of long-time service. Accordingly, in order to prevent it, a high-hardness material such as SUS stainless steel (Vickers hardness: about 180) is often used as a material for developing sleeves. Formerly, alundum blasting making use of alumina particles as blasting abrasive grains is also known (Japanese Patent Application Laid-Open No. 57-66455).
However, as disclosed in Japanese Patent Applications Laid-Open No. 57-116372, No. 58-11974 and No. 1-131586, in the blasting making use of alundum, rough surface with sharp unevenness is formed at the developing sleeve surface made of SUS stainless steel. FIG. 2 diagrammatically shows a roughness profile curve of a developing sleeve surface having been subjected to alundum blast treatment. It is known that, during its long-term service, toner particles and so forth having especially fine particle size are buried in sharp valleys of this surface (hereinafter this state in which the toner particles and so forth are buried is called “sleeve contamination”) and the charging of toner is obstructed at that part to cause faulty images.
For example, a method is designed in which the blast treatment is made using spherical particles such as glass beads. FIG. 3 diagrammatically shows a like roughness profile curve obtained in the glass beads blast treatment. As shown in FIG. 3, according to the glass beads blast treatment, rough surface with a gentle profile form can be obtained at the surface of the developing sleeve made of SUS stainless steel. Thus, the sleeve contamination can be lessened, though not sufficient, to a certain level.
It is becoming prevailing to use aluminum as a material for developing sleeves. Although the SUS stainless steel is expensive, there is an advantage that the use of aluminum enables cost reduction of developing sleeves.
However, the aluminum sleeve has a hardness as low as Hv of about 100, and hence the surface unevenness may easily become worn as a result of use, so that the unevenness may lessen at an early stage.
In more recent years, in order to achieve a higher image quality, there is a tendency of making toners have much smaller particle diameter. This has proved to tend to cause the sleeve contamination much more than ever.
This is explained with reference to FIG. 4. FIG. 4 is an enlarge view of the unevenness corresponding to the roughness profile curve shown in FIG. 3. FIG. 3 shows, as described above, a roughness profile curve obtained when the surface of the SUS stainless-steel developing sleeve is subjected to the blast treatment with spherical-particle glass beads. In the profile shown in FIG. 4, in the case of toners with a large particle diameter, any particles do not enter any cracks in large hills and dales in the roughness profile curve, namely, do not enter small valleys as exemplified by valleys a, b and c. However, with an decrease in particle diameter of the toner, toner particles entering the small valleys a, b and c may increase to cause sleeve contamination, as so considered.
For example, small-diameter toner particles having a particle size distribution of about 7 μm in volume-average particle diameter commonly contain about from 15% by number to about 20% by number of small toner particles having particle diameter of 4 μm or less. Such particles enter the small valleys a, b and c. Of course, any finer powder in toner may be cut away in order to lessen smaller toner particles, but it is impossible under the existing conditions to remove them completely.
As stated previously, even without making toners have smaller particle diameter, charge obstruction on toner also tends to occur because of even a slight sleeve contamination when toners having a low chargeability are used, bringing about difficulties such as density loss.
In another case of a developer to which an external additive having the same triboelectric series as its toner has been added, what is called “sleeve ghost”, which is a history of print patterns, may appear on the developing sleeve, and this may also appear on printed images. This sleeve ghost has a tendency that, the higher charging performance the external additive has, the more easily it appears. For example, a sleeve ghost which may appear in the case of a developer obtained by adding negatively chargeable fine particles externally to a negatively chargeable toner turns a positive ghost. More specifically, density variation (unevenness) occurs between the part (X) where only thin development is performed because unprinted areas (white background) had continued and the part (Y) where thick development is performed because the printing had been continued.
Think about the mechanism of how this sleeve ghost forms. In the developing step, the toner charged anew electrostatically is fed to areas where the developer (toner) has been consumed on the developer-carrying member (developing sleeve), and the next development is performed there. At this stage, charge quantity differs between the toner remaining on the developing sleeve without being consumed and the toner fed anew. The toner having higher charge quantity has a higher ability to fly to the electrostatic latent image on the latent-image-bearing member, but at the same time shows a tendency of being electrostatically strongly bound to the developing sleeve because of the mirror force acting between the toner and the developing sleeve. Thus, the ability of development depends on the balance between the ability to fly and the mirror force.
This sleeve ghost is also deeply concerned in a layer which is formed by a fine powder contained in the toner present on the developing sleeve and an external additive added externally to the toner. Namely, the reason is that the toner which forms the lowermost layer of the toner layer on the developing sleeve come to differ clearly in particle size distribution between the toner-consumed areas and the toner-unconsumed areas, so that a fine-powder layer which is formed by the fine powder contained in the toner present on the developing sleeve and the external additive added externally to the toner is formed at the lowermost layer of the toner present at the toner-unconsumed areas. The particles which form this fine-powder layer have a large surface area per volume, and hence, compared with the toner having large particle diameter, have a large quantity of triboelectrically generated electric charges per unit weight, so that such particles are electrostatically strongly bound to the developing sleeve because of their own mirror force. Hence, the toner present above the part where this fine-powder layer has been formed comes to have a low developability because it is not sufficiently triboelectrically charged with the developing sleeve surface, so that this may appear as the sleeve ghost on images.
In general, when the toner anew charged electrostatically and fed to the toner-consumed areas has a higher developability than the toner remaining on the developing sleeve without being consumed, the above positive ghost appears. On the contrary, when the toner anew charged electrostatically and fed to the toner-consumed areas has a lower developability than the toner present at other areas, a negative ghost appears, contrary to what is shown in FIG. 5, such that the areas at which the toner has been replaced because the printing had been continued come to have a lower density than the areas at which any toner has not been replaced because the unprinted areas (white background) had continued.
The sleeve ghost explained above is a phenomenon which occurs because the charging of the toner greatly depends on the triboelectric charging with the developing sleeve, together with the formation of the fine-powder layer comprised of the fine powder contained in the toner and the external additive added externally to the toner. Accordingly, in order to solve the problem of sleeve ghost, the mirror force acting between the developing sleeve and the charged-up fine-powder toner present in the vicinity of the developing sleeve surface must be removed or be made smaller by any means.
Besides the above phenomenon of sleeve ghost, a problem may arise such that areas having a low density occur in vertical lines on images obtained by development. More specifically, this is a phenomenon that character lines become slender in the case of character images, and density becomes low in the case of halftone images and solid black images.
This phenomenon is called “fading”. We have observed the developing sleeve on the occasion that this fading has occurred, to find that a toner layer with a uniform thickness has been formed on the sleeve. However, upon measurement of the quantity of triboelectrically generated electric charges of the toner on the sleeve, it has been ascertained that the quantity of electric charges of the toner at the region corresponding to the low-density vertical lines in images has a lower value than a normal value.
The reason why the charge quantity of the toner lowers partly as described above is presumed as follows: copied images or image output patterns are not necessarily uniform in image planes, so that areas where the toner is consumed in a large quantity and areas where it is consumed in a small quantity may come. Of these, at the areas where the toner is consumed in a small quantity, the toner is replaced in a relatively small quantity. Hence, the circulation of the toner in the vicinity of the developing sleeve at the corresponding areas is obstructed, so that the toner comes to be packed in the vicinity of the sleeve. Then, in this state the toner is rubbed with the sleeve surface, where the toner particles may deteriorate to become unable to be triboelectrically charged in a normal condition. As the result, continuing copying or printing in this state accelerates the deterioration of the toner to cause a decrease in density (density loss) at such areas.
The low charged toner also passes through the developer layer thickness regulation zone, as a layer having a thickness equal to that of the normally charged toner layer, by the force of friction with the sleeve. Hence, the thickness of the toner layer is uniform on the sleeve.
The smaller the toner particle diameter is, the more the fading is liable to occur. This is due to the fact that a fine-particle toner is highly agglomerative. More specifically, this is because the fine-particle toner has small particle diameter, and, compared with toners having usual particle diameter, has so large a surface area as to be triboelectrically charged in excess, to cause a decrease in fluidity of the toner as a result of electrostatic agglomeration. Moreover, the external additive standing adhered to toner particles and the vicinity thereof also has a great influence. Accordingly, care must be taken when any particles that may obstruct the fluidity of toner or particles that may greatly change the charge quantity of toner are added.
The fading may also remarkably occur not only in a low-humidity environment in which the decrease in fluidity due to the electrostatic agglomeration of toner is accelerated, but also in a normal-temperature and normal-humidity environment or in a high-temperature and high-humidity environment in which the chargeability of toner lowers.
Thus, although approach has been made from both developers (toners) and developer-carrying members in order to establish the image-forming method having the step of injection charging, the cleaning-at-development image-forming method or the cleanerless image-forming method, any proposal has not been made until now in respect of a system in which the problems having been discussed above have all been solved. Under existing circumstances, studies have not yet sufficiently been made.